1. Field of the Invention
The present invention relates to a resonant circuit used for the prevention of shoplifting or the like, and more particularly, to a resonant circuit having a capacitor formed on a flexible substrate wherein the capacitor is deactivated at a dimpled area by exposure to a predetermined voltage level.
2. Description of Related Art
In retail shops, libraries or the like, a surveillance system including a resonant tag that resonates with a radio wave, a transmitting antenna and a receiving antenna has been used for the prevention of shoplifting. In an embodiment, the resonant tag is composed of an insulating film, a coil and a plate made of a conductive metal foil formed on one side of the insulating film, and a plate made of a conductive metal foil formed on the other side, which constitute an LC circuit and resonates with a radio wave at a particular frequency.
If an article with the resonant circuit attached passes through a surveillance area without being disabled at checkout, the resonant circuit resonates with the radio wave from the transmitting antenna, and the receiving antenna detects the resonance and generates an alarm. A typically used resonant frequency is 5 to 15 MHz, because frequencies within the range can be easily distinguished from various noise frequencies. In electronic article surveillance (EAS), a frequency of 8.2 MHz is most popularly used, and in radio frequency identification (RFID), a frequency of 13.56 MHz is most popularly used.
By way of example only, FIGS. 1-3 depict a prior art LC resonant circuit in the form of a tag 10 which includes a coil 11 and a first capacitor plate 12 on one side (FIG. 1) of a substrate 13 and a second capacitor plate 14 on the other side of the substrate 13 (FIG. 2). FIG. 3 is a cross-sectional view of this prior art tag showing a typical substrate thickness, t, of approximately 20 microns, which tends to be the thinnest dielectric that can be formed using conventional dielectric forming methods (e.g., extruding polyethylene between the metal layer). Adhesive layers 15 and 17 secure the metal layers to the substrate 13 respectively.
Prior art resonant tags formed as in FIGS. 1-4 are commonly deactivated, once an article with the resonant tag is purchased, by application of a predetermined voltage to the tag. The tag typically has a thinned part of the dielectric (FIG. 4: 10a, 10b) commonly referred to as a dimple. The dimple provides a shorter distance between the tip of the dimple and the opposing plate, than between the remaining surfaces of the two plates. When a high level of electromagnetic energy is applied to the tag, a voltage in excess of the breakdown voltage can be created between the tip of the dimple and the opposing plate. This causes the dielectric material to break down, thereby substantially short circuiting the two plates to each other. When the capacitor shorts out in the weakened area, its capacitance goes substantially to zero and the resonant frequency of the tag is moved out of the range of frequencies being swept by the detection equipment. Such a dimple for deactivating a resonant tag is disclosed in U.S. Pat. No. 5,142,270, entitled “Stabilized Resonant Tag Circuit and Deactivator,” issued to Appalucci et al. on Jul. 8, 1992.
One problem with the known methods for deactivating tags is that a tag may spontaneously reactivate at a later time. It is believed that one reason why tags reactivate may be that the short circuit between the plates of the capacitor is formed by fragile dendritic structures created by the breakdown of the dielectric. The structures providing the short circuit between the plates can therefore break at a later time, for example, due to flexing of the tag, and restore the high resistance path between the plates. When this occurs, a security tag that is deactivated after a legitimate purchase can set off an alarm if an innocent bearer of the tag inadvertently brings it into a detection region. This problem commonly occurs when the tag is attached to an article of clothing and not removed by the purchaser before wearing the clothing. Flexure of the tag in normal wear of the clothing and in washing can cause the tag to reactivate due to damage to the dendritic structures.
In resonant tags having polyethylene dielectrics, as many as 50% of the tags become reactivated with wearing or laundering. This unintended reactivation has undesirable consequences for the wearer of the clothing, who will activate security tag detection devices when entering or exiting any store with equipment tuned to the tag's resonant frequency. Not only is the false alarm inconvenient and embarrassing for the person wearing the clothing with the reactivated tag, but frequent false alarms can cause a “boy who cried wolf” effect. Store personnel can become lax about enforcement of tag alarms when many of them are falsely triggered by reactivated tags on legitimately purchased goods. The inconvenience and embarrassment of false alarms may so irritate consumers that sales of clothing brands bearing re-activatable tags are lost.
Thus a need exists for an improved resonant circuit with a capacitor that incorporates a dimple to form a shorted area when the tag is disabled, wherein the shorted area does not later return to its original state, thereby returning the tag to a functioning resonant circuit under physical distortion of the tag or temperature swings.